1. Field of the Invention
The present invention relates to a method of multiplex-driving active matrix liquid-crystal display (LCD) elements.
2. Description of the Related Art
LCD elements are used in television sets, personal computers, and the like.
Each of these displays comprises a plurality of active matrix LCD elements. These active matrix LCD elements are arranged in rows and columns, each comprising a pixel and an active element. The active elements can drive the pixels in high time-division fashion, without causing crosstalk among the pixels.
Active matrix LCD elements are classified into two types. The first type comprises a pixel and a two-terminal active element, e.g., a nonlinear resistive element (more specifically, a thin-film diode (TFD), for example). The second type comprises a pixel and a three-terminal active element, e.g., a thin-film transistor (TFT).
Active matrix LCD elements of the first type (hereinafter referred to as "TFD LCD elements"), whose active elements are thin-film diodes, are classified into two types. The first-type TFD LCD elements have so-called "diode-ring structure." The second-type TFD LCD elements have so-called "back-to-back structure."
FIG. 1 is a plan view of a liquid-crystal display comprising TFD LCD elements having the diode-ring structure. More precisely, it shows only four of the TFD LCD elements incorporated in the display and arranged in rows and columns. The liquid-crystal display has a pair of transparent substrates (not shown), a liquid-crystal layer (not shown) sandwiched between the substrates, a plurality of pixel electrodes 1, a plurality of active elements 2 (i.e., thin-film diodes), a plurality of signal lines 3, and a plurality of opposing electrodes 4. The pixel electrodes 1 are formed on the first substrate and arranged in rows and columns. The active elements 2 are mounted on the first substrate and arranged in rows and columns. The signal lines 3 extend parallel to the rows of the TFD LCD elements, for supplying drive signals to the rows of active elements 2. The opposing electrodes 4 are formed on the second transparent substrate and oppose the pixel electrodes 1.
The opposing electrodes 4 extend parallel to the columns of pixel electrodes 1, respectively. Hence, each pixel of the liquid-crystal display shown in FIG. 1 comprises a pixel electrode 1, that portion of an opposing electrode 4 which overlaps the pixel electrode 1, and that portion of the liquid-crystal layer (not shown) which is interposed between the pixel electrode 1 and said portion of the opposing electrode 4.
Each active element 2 is a so-called "diode ring" comprising two diodes 5 and 6 which are connected in parallel and orientated in the opposite directions. As is evident from FIG. 1, the active element 2 is connected at one end to the pixel electrode 1, and at the other end to the signal line 3.
Any TFD LCD element of the liquid-crystal display is driven in time-division fashion. A scan signal is supplied to the signal lines 3, and the data signal is supplied to the opposing electrodes 4.
More specifically, the diode ring 2 (i.e., the active element) is turned on or off by the voltage applied between its input terminal and the opposing electrode 4. (The input terminal of the active element 2 is the node where the element 2 is connected to the signal line 3, and said voltage is the potential difference between the scan signal and the data signal.) When the active element 2 is turned on, an electric charge is accumulated between the pixel electrode 1 and the opposing electrode 4 which opposes the pixel electrode 1. The charge, thus accumulated, drives that portion of the liquid-crystal layer which is interposed between the pixel electrode 1 and the opposing electrode 4, whereby the pixel displays the data corresponding to the data signal.
With reference to FIG. 2A, it will now be explain how to drive one of the pixels of, for example, the second row (hereinafter called "selected pixel"). FIG. 2B shows the waveform of a scan signal S.sub.S to be supplied to the signal line 3 to which the pixel is connected, and that of a data signal S.sub.D to be supplied to the opposing electrode 4 which is part of the selected pixel. In this figure, T.sub.S is a-selecting period during which the row of pixels, including the selected pixel, is selected, and T.sub.O is a non-selecting period during which the other rows of pixels are selected. The selecting period T.sub.S is obtained by dividing a one-field time T.sub.F by the number of pixel rows provided (i.e., the number of signal lines 3).
When the scan signal S.sub.S is supplied to the signal line 3 to which the pixels of the second row are connected, and the data signal S.sub.D is supplied to the opposing electrode 4 which is part of the selected pixel, a voltage Va, which changes as is shown in FIG. 3, is applied between the pixel electrode 1 of the selected pixel and that portion of the opposing electrode 4 which overlaps this pixel electrode 1. As is evident from FIG. 3, this voltage Va is a difference between the voltages of the scan signals S.sub.S and S.sub.D. The value V1 which the voltage Va has during the selecting period T.sub.S is higher than the threshold voltage of the diode ring 2. The value V3 which the voltage Va has during the non-selecting period T.sub.O is lower than the threshold voltage of the diode ring 2.
The selected pixel, which is formed of a pixel electrode 1, that portion of an opposing electrode 4 which overlaps the pixel electrode 1, and that portion of the liquid-crystal layer which is interposed between the electrode 1 and said portion of the opposing electrode 4, is equivalent to a capacitor. The diode ring 2 remains off during the non-selecting period T.sub.O. Hence, the voltage V1 between the input of the diode ring 2 and the opposing electrode 4 is applied across the diode ring 2 during the selecting period T.sub.S.
The diode ring 2 has the current-voltage (I-V) characteristic illustrated in FIG. 4. As is evident from FIG. 4, when the voltage applied to the diode ring 2 rises above the threshold voltage of the diode ring 2 at the start of the selecting period T.sub.S, the diode ring 2 is turned on. As a result, a current flows through the ring 2, whereby an electric charge is accumulated in the equivalent capacitor, i.e., the selected pixel. As the pixel is charged more and more, the voltage Va across the diode ring 2 decreases gradually. At the end of the selecting period T.sub.S, or at the start of the non-selecting period T.sub.O, the voltage Va falls to V2 which is lower than the threshold voltage of the diode ring 2. Hence, the diode ring 2 is turned off. The selected pixel holds the electric charge accumulated during the selecting period T.sub.S.
The voltage V.sub.LC, which is applied between the pixel electrode 1 and the opposing electrode 4 which form selected pixel, changes as is illustrated in FIG. 5. More precisely, the voltage V.sub.LC gradually increases during the selecting period T.sub.S as the pixel is increasingly charged. It falls at the end of the selecting period T.sub.S, and remains unchanged during the non-selecting period T.sub.O by virtue of the charge accumulated during the selecting period T.sub.S.
Thus far it has been described how the selected pixel of the second row is driven. Any other pixel of any other row of the liquid-crystal display shown in FIG. 1 is driven in the same way, whenever it is selected. As scan signals S.sub.S are sequentially supplied to the signal lines 3, and data signals S.sub.D are sequentially supplied to the opposing electrodes 4, the pixels are sequentially selected and driven, accumulating charges corresponding to the data signals. Due to the electric charges they have accumulated, the pixels have their transmittances changed, thus displaying the image represented by the data signals.
Described above is how TFD LCD elements having the diode-ring structure are selected and driven in time-division fashion, in order to display an image. The TFD LCD elements having the back-to-back structure are selected and driven in time-division fashion, by the same method as has been described above. TFD LCD elements have no crosstalk among them and can, therefore, be driven in high time-division fashion, no matter whether they have the diode-ring structure or the back-to-back structure.
A liquid-crystal display having active matrix LCD elements of the second type (hereinafter referred to as "TFT LCD elements"), whose active elements are thin-film transistors, will now be described. Though not shown in any drawing attached hereto, this liquid-crystal display has a pair of transparent substrates, a liquid-crystal layer sandwiched between the substrates, a plurality of pixel electrodes arranged on the first substrate in rows and columns, a plurality thin-film transistors (TFTs) arranged on the first substrate and having sources connected to the pixel electrodes, respectively, a plurality of scan signal lines for supplying scan signals to the gates of the TFTs, a plurality of data lines for supplying data signals to the drains of the TFTs, and a plurality of opposing electrodes arranged parallel on the second substrate. In this liquid-crystal display, each of the pixels comprises a pixel electrode, that portion of an opposing electrode which overlaps the pixel electrode, and that portion of the liquid-crystal layer which is interposed between the pixel electrode and said portion of the opposing electrode.
The TFT LCD elements are sequentially driven in time-division fashion as scan signals are sequentially supplied to the rows of TFTs and data signals are supplied to the columns of TFTs in synchronism with the scan signals, while a reference voltage is being applied to the opposing electrodes.
Each of the TFTs is turned on when a scan signal is supplied to its gate. Then, a current proportional to the voltage of the data signal supplied to the drain of the TFT flows to the pixel electrode. An electric charge is thereby accumulated between the pixel electrode and the opposing electrode which overlaps the pixel electrode. Due to the charge, thus accumulated, that portion of the liquid-crystal layer which is interposed between the pixel electrode and the opposing electrode has its transmittance changed. As a result, the pixel displays a dot represented by the data signal.
The electric charge is held between the pixel electrode and the opposing electrode during the non-selecting period. In other words, the charge is held there while any other row of pixels is being selected. When the next data signal is supplied to the drain of the TFT whose source is connected to the pixel electrode, the charge corresponding to this data signal is accumulated between the pixel electrode and the opposing electrode.
Hence, as the TFT LCD elements are sequentially driven in time-division fashion as described above, they display, in cooperation, an image consisting of the dots represented by the data signals supplied to the drains of the TFTs.
Like the TFD LCD elements, the TFT LCD elements have no crosstalk among them, Therefore, they can be driven in high time-division fashion.
The active matrix LCD elements described above have an active element each, which is a thin-film semiconductor elements, such as a TFD or a TFT. A great capacitance is built up between the electrodes of the semiconductor element (i.e., the two electrodes of a TFD, or the gate electrode and source or drain electrode of a TFT).
Each active matrix LCD element is represented by the equivalent circuit of FIG. 6(a), which comprises a pixel capacitor C.sub.LC (i.e., the capacitance of a pixel) and an active element 2 connected in series to the capacitor C.sub.LC. Once the active element 2 is turned off, the active matrix LCD element is represented by the equivalent circuit of FIG. 6(b), which comprises the pixel capacitor C.sub.LC and an element capacitor C.sub.D (i.e., the capacitance of the active element 2). The element capacitor C.sub.D is connected in series to the pixel capacitor C.sub.LC.
Therefore, as is shown in FIG. 5, the inter-electrode voltage V.sub.LC of the active matrix LCD element increases to the voltage applied between the signal line 3 and the opposing electrode 4 during the selecting period T.sub.S when the active element 2 remains on. When the active element 2 is turned off at the start of the non-selecting period T.sub.O, the voltage V.sub.LC decreases since it is divided into two parts which correspond to the pixel capacitance C.sub.LC and the element capacitance C.sub.D, respectively. How much the voltage V.sub.LC falls depends on the ratio of the element capacitance C.sub.D to the pixel capacitance C.sub.LC.
More specifically, the voltage V.sub.LC applied between points b and c in FIG. 6(b) is given: EQU V.sub.LC =Va.multidot.C.sub.D /(C.sub.LC +C.sub.D)
where Va is the voltage applied between points a and c. Obviously, the voltage V.sub.LC decreases greatly during the non-selecting period T.sub.O, if the element capacitance C.sub.D is greater than the pixel capacitance C.sub.LC.
In each active matrix LCD element, that portion of the liquid-crystal layer which is sandwiched between the electrodes 1 and 4 is driven actually by the voltage applied between these electrodes during the non-selecting period T.sub.O which is much longer than the selecting period T.sub.S. Hence, the voltage for driving said portion of the liquid-crystal layer will inevitably decrease if the voltage V.sub.LC falls greatly at the start of the non-selecting period T.sub.O.
In order to apply a sufficiently high voltage to the liquid-crystal layer, it is necessary to increase the voltage applied between the signal line 3 and the opposing electrode 4. To this end, a high-voltage drive circuit must be used, which consumes much electric power.
The reduction of the inter-electrode voltage of each pixel can be minimized if the ratio of the element capacitance C.sub.D to the the pixel capacitance C.sub.LC is small. To decrease the capacitance C.sub.D, thereby to make the ratio C.sub.D /C.sub.LC sufficiently small, it would suffice to use a pair of thin-film diodes or a thin-film transistor as active element 2, which has a small area. If the thin-film diodes or the thin-film transistor, used as active element 2, has so small an area that the ratio C.sub.D /C.sub.LC is about 0.1 or less, the voltage V.sub.LC, which is applied between points b and c shown in FIG. 6(b), will decrease, but not so much. As a result, the voltage applied to the liquid-crystal layer is high enough to drive the pixel. Hence, the active matrix LCD element can be driven with a relatively small amount of electric power.
In order to manufacture a thin-film diode or transistor having a small area, however, high-precision patterning needs to be accomplished. It is difficult to achieve such high-precision patterning, making it hard to form a thin-film diode or transistor having an small area and, hence, a negligibly small capacitance. Inevitably, the inter-electrode voltage V.sub.LC of the pixel will decrease due to the capacitance C.sub.D of the active element 2 at the start of the non-selecting period T.sub.O. Even if the capacitance C.sub.D is somewhat small, a voltage must be applied between the input of the active element 2 and the opposing electrode 4, which is high enough to compensate for the reduction in the inter-electrode voltage V.sub.LC which occurs at the beginning of the non-selecting period T.sub.O.
The conventional method of driving active matrix LCD elements has another problem. The pixel of each LCD element has its transmittance changed too much even if the drive voltage applied between the input of the active element 2 and the opposing electrode 4 is as high as the inter-electrode voltage V.sub.LC of the pixel. The problem will be detailed, with reference to FIG. 7.
FIG. 7 represents the voltage-transmittance (V-T) characteristic of the pixel of an active matrix LCD element, which has a diode-ring used as active element, when driven in time-division fashion by the conventional method. In this figure, curve I indicates the V-T characteristic the pixel has when all other pixels of the same column (i.e., all other pixels opposing the same opposing electrode 4) are driven to allow light to pass through them. Curve II in FIG. 7 indicates the V-T characteristic the pixel has when all other pixels of the same column are driven to inhibit light from passing through them. Both V-T characteristics illustrated in FIG. 7 are inherent in the pixels of LCD elements incorporated in a liquid-crystal display which has two polarizing plates arranged with their polarization axes crossing at right angles.
As is evident from FIG. 7, the transmittance of the pixel of each active matrix LCD element changes in accordance with whether the other pixels of the same column are driven to allow or inhibit the passage of light, even though the drive voltage applied to the active matrix LCD element remains unchanged. This is because the inter-electrode voltage V.sub.LC of the pixel is changed by the data signal supplied to all other pixels of the same column during the non-selecting period T.sub.O. In other words, the data signal applied to the other pixels of the same column imposes a great influence on the V-T characteristic of the pixel, greatly changing the transmittance of the pixel.
With the conventional method of driving active matrix LCD elements, it would be difficult to control vary the brightnesses of the individual pixels, thereby to accomplish gray-level control. The conventional method can hardly help to display gray-scale images.